The present invention relates to a reception station and a transmission diversity control method, and more particularly to a reception station that issues requests to start and stop transmission diversity to a base station having a transmission diversity function, and communication system and a transmission diversity control method for the reception station.
In a best effort type communication system, communication speed increases as reception performance improves. A transmission diversity control as well as an equalization control is applied for improving the reception performance. The transmission diversity control enables a mobile station to obtain a diversity effect with a single antenna, and includes an open loop mode in which feedback is not required and a closed loop mode requiring feedback.
Open Loop Transmission Diversity System
Space Time Transmit Diversity (STTD) is an open loop mode system in which a set of two encoded symbols are transmitted on respective antennae and a transmission power of each antenna is ½ the normal power (Non-patent Document 1). As shown in FIG. 18, in STTD, an STTD encoder 1 on a transmission side converts two continuous symbol data [x0, x1] of a period T into two symbol data sequences. A first data sequence is [x0, −x1*] and a second data sequence is [x1, x0*]. As shown in FIG. 19, the two data sequences are transmitted to a reception antenna ATr by two transmission antennae ATt0, ATt1. Assuming that channel response characteristics between the two transmission antennae ATt0, ATt1 and the single reception antenna Atr are h0, h1, antenna reception signals at times t and t+T are r0, r1, respectively, and noise values are n0, n1, the antenna reception signals r0, r1 can be expressed respectively by the following equations.r0=h0x0+h1x1+n0 r1=−h0x1*+h1x0*+n1 
As shown in FIG. 20, an STTD reception unit includes a channel estimation unit 2, an STTD decoder 3, and a decoding unit 4. The channel estimation unit 2 estimates the channel response characteristics h0, h1 and inputs the estimated characteristics into the STTD decoder 3. The STTD decoder 3 outputs signals s0, s1 shown respectively in the following equations.
                              s          0                =                ⁢                                            h              0                        ⁢                          r              0                                +                                    h              1                        ⁢                          r              1              *                                                              =                ⁢                                            (                                                                                                              h                      0                                                                            2                                +                                                                                                h                      1                                                                            2                                            )                        ⁢                          x              0                                +                                    h              0                        ⁢                          n              0                                +                                    h              1                        ⁢                          n              1              *                                                                        s          1                =                ⁢                                            h              1                        ⁢                          r              0                                -                                    h              0                        ⁢                          r              1              *                                                              =                ⁢                                            (                                                                                                              h                      0                                                                            2                                +                                                                                                h                      1                                                                            2                                            )                        ⁢                          x              1                                +                                    h              1                        ⁢                          n              0                                -                                    h              0                        ⁢                          n              1              *                                          
The decoding unit 4 then performs decoding using the output signals from the STTD decoder 3. Note that the values in parentheses in the above equations are STTD gain values.
Closed Loop Transmission Diversity System
FIG. 21 is an illustrative view of a closed loop transmission diversity system. In a closed loop transmission diversity system, a wireless base station of a cellular mobile communication system is provided with a plurality of antenna elements and executes the following operations (1)-(3) repeatedly: (1) different amplitude and phase control is implemented on identical transmission data signals on the basis of feedback information FBI transmitted from a mobile station, (2) a pilot signal is multiplexed with the transmission data subjected to the amplitude and phase control and then transmitted on the plurality of antennae, and (3) the transmission data are received on the mobile station side, whereupon the feedback information (amplitude and phase control amounts) is determined again using a down pilot signal, multiplexed with an up channel signal, and transmitted to the base station side.
As shown in FIG. 21, in closed loop transmission diversity for W-CDMA, which is a third generation mobile communication system, a system employing two transmission antennae is used. In the drawing, orthogonal pilot patterns P1, P2 are generated in a pilot signal generation unit 11, incorporated into transmission data signal in combiners units CB1, CB2, and transmitted from transmission antennae 10-1, 10-2, respectively. A channel estimation unit (not shown) on a mobile station reception side estimates channel impulse response vectors h1, h2 from the respective transmission antennae 10-1, 10-2 of the base station to a mobile station reception antenna 12 by obtaining a correlation between the received pilot signal and a known corresponding pilot pattern.
A weight calculation unit 13 uses the channel estimation values to calculate amplitude and phase control vector (weight vector w, w=[w1, w2]T) of the respective transmission antennae 10-1, 10-2 of the base station so that a power P shown in a following Equation (1) reaches a maximum. The calculated vector is then quantized, multiplexed with an up channel signal, and transmitted to the base station side as the feedback information FBI. Note that it is not necessary to transmit both of the values w1, w2, and when w1=1 is determined, the value w2 may be transmitted alone.P=wHHHHw  (1)H=[h1, h2]  (2)
Here, h1, h2 are the channel impulse response vectors obtained respectively from the antenna 10-1 and the antenna 10-2. Further, the superscript suffixes in HH and wH indicate that Hermite conjugates of H and w are taken.
In the mobile station, weight coefficients w1, w2 (the weight vector w) are calculated by the weighting calculation unit 13 as described above, the weight coefficients w1, w2 are multiplexed with up transmission data by a multiplexing unit 18 to generate the feedback information FBI, and the feedback information FBI is transmitted to the base station from the transmission antenna 14. In the 3rd Generation Partnership Project (3GPP), the FBI information is transmitted to the base station after being mapped on an uplink DPCCH (Dedicated Physical Control Channel).
In the base station, the feedback information from the mobile station is received by a reception antenna 15, a feedback information extraction unit 16 extracts the weight coefficients w1, w2 serving as control amounts, and an amplitude/phase control unit 17 multiplies down transmission data by the weight coefficients w1, w2 using multipliers MP1, MP2 and performs amplitude and phase control on signals to be transmitted from the transmission antennae 10-1, 10-2. As a result, the mobile station can receive signals transmitted from the two diversity transmission antennae 10-1, 10-2 efficiently. Note that ideally, the signals transmitted from the two diversity transmission antennae 10-1, 10-2 reach the reception antenna of the mobile station in phase.
With recent demands for improvements in communication speed, communication devices having as linear equalizers, which achieve an improvement in reception performance by employing a signal correlation to reduce multipath interference, have been developed and are beginning to grow in popularity. Meanwhile, in STTD, which is an open loop transmission diversity mode, a signal correlation is not generated between the transmission antennas, and therefore it is well-known that STTD disturbs an improvement in reception performance achieved by an equalizer. In other words, when STTD is combined with an equalizer, an inter-path correlation value is half of its normal value due to the effect of the STTD, and as a result, the effect of the equalizer is reduced. Therefore, it has been learned by the inventors that when an equalizer is used, the reception performance can be improved by stopping transmission diversity STTD.
Incidentally, depending on the environment, the effect of an equalizer may be large or small. Ideally, the reception performance is improved by stopping transmission diversity in an environment where the effect of the equalizer is large and starting transmission diversity in an environment where the effect of the equalizer is small. Conventionally, however, control to start transmission diversity or stop transmission diversity is not performed during an operation. The reason for this is that with a RAKE system for a conventional communication system employing CDMA, performance deterioration due to the application of transmission diversity does not occur. Thus, a base station employing transmission diversity STTD cannot extract sufficient performance from an equalizer installed in a mobile station.
In a first conventional technique, a reception mode of a mobile station is switched between a TSTD (Time Switched Transmission Diversity) mode and a non-TSTD mode in accordance with a transmission mode of a base station (Patent Document 1). More specifically, a message indicating whether the transmission mode is the TSTD mode or the non-TSTD mode is transmitted from the base station to the mobile station, and upon reception of the message, the mobile station switches the reception mode. A second conventional technique provides a transmission diversity system for realizing a transmission diversity effect even on a transmission path exhibiting large delay dispersion (Patent Document 2). In a third conventional technique, an open loop transmission diversity function and a closed loop transmission diversity function are provided, and these functions are switched in accordance with predetermined conditions (Patent Document 3). In this conventional technique, the closed loop transmission diversity is normally used to eliminate a multipath environment, but when the reliability of a feedback signal decreases due to a high Doppler rate or the like, the multipath environment is eliminated after switching to the open loop transmission diversity.
However, none of these conventional techniques can be used to start and stop transmission diversity control in the base station during an operation in order to extract a sufficient performance from the equalizer installed in the mobile station.